Exploring the Adsorption and Reactions of Methyl Radicals on M(111) Surfaces (M=Cu, Ag, Au): A DFT Study

It was reported that adsorbed methyl radicals produce ethane with Ag⁰- and Au⁰-nanoparticles in aqueous media, whereas on Cu⁰-powders, the product is methanol. The source of these differences was explored computationally, using the DFT method. The results indicate that up to six radicals can be adsorbed on Ag(111) and Au(111), (top site), while only four can be adsorbed on Cu(111) (fcc site), each surface containing eight atoms. The diffusion of the radicals on the surface is very easy on silver and copper, as this is achieved with a very low barrier (0.06 eV and 0.15 eV for Ag(111) and Cu(111), respectively), while on Au(111), the barrier is higher, 0.51 eV. The formation of ethane via a reaction of two adsorbed radicals is thermodynamically plausible for all studied coverage ratios on the three surfaces, but kinetically, it is plausible at room temperature only on Au(111) and Ag(111) at full coverage. Ethane can also be produced on Au(111) and Ag(111) by a collision of a solvated radical and an adsorbed radical. This is a barrierless process. On Cu(111), the yield of such a process is CH₄(aq), and an adsorbed CH₂ which reacts further with a non-adsorbed water molecule to produce adsorbed CH₃OH.